Raw wood must be seasoned to be used as a lumber product and the are three conventional process methods of seasoning lumber as follows;
(1) The tunnel kiln method of seasoning lumber used in the Continent of America, wherein hot air is blown on lumber, and dischanged from the both ends of the tunnel. PA1 (2) The method of seasoning lumber using a hot air dryer after prolonged natural seasoning. This method is used in Europe, Japan, and many other countries. PA1 (3) The lumber seasoning method known as steaming and vacuum method, which uses a pressure vessel and repeats pressurizing and heating with saturated steam and reducing pressure to less than the atmospheric pressure.
According to process (1) described above, enormous loss due to cracks and warps occurs and moisture content doesn't become uniform. Moreover when the moisture content falls and the contained water becomes difficult to evaporate, this method cannot enhance reduction of moisture content, resulting in a disadvantage whereby the tunnel kiln must be greatly lengthened.
On the other hand, method (2) described above, is a common lumber seasoning method used in many areas of the world. The greatest possible care is taken to operate the hot air wood dryer so as to reduce loss due to cracks and warps in the lumbar. However, much loss due to cracks and warps occurs during natural seasoning before artificial seasoning, and such problems are regarded as inevitable and have not yet been solved.
Moreover, method 3 described above is a lumber seasoning method which repeats the cycles, and includes the process of heating and pressurizing with saturated steam, and also the process of reducing the pressure below the atmospheric pressure. Thereby depreciation costs a great deal and moisture content within lumber cannot be reduced below 20%. The reason is, during the process of pressurizing and heating, the water contained within lumber is forced to move into the core portion, thereby low-temperature water lump arises. Since the specific heat of the water is three times as large as that of the lumber, even if the time of pressurizing and heating is extended as long as possible, the temperature within said lumber doesn't become uniform. In the case of reducing pressure while the temperature remains non-uniform, the contained water boils and evaporates only from the surface of the lumber. Therefore taking the quantity of heat into account, even if the time of reducing pressure below the atmospheric pressure is extended, the contained water does not boil, resulting in absorption of the evaporated water from the surface of the material. Therefore, the above-mentioned condition equals to that of reducing the contained water with the hot air wood dryer, and a pressurizing and heating process and a pressure reducing process must be repeated many times with the result that method 3 has fallen into disuse due to the expense of seasoning the lumber.